Project Summary/Abstract Lung cancer is the leading cause of cancer deaths worldwide and there is an urgent need for effective treatment. Comprehensive genetic profiling has revealed that the LKB1 (STK11) tumor suppressor gene is frequently altered in non?small cell lung cancer (NSCLC), a major form of lung cancer. Lung cancer with LKB1 inactivation has distinct biology and behaviors; however, no targeted therapies are currently available for this unique, prevalent molecular subtype of lung cancer. While it is traditionally challenging to target an inactive or absent tumor suppressor, its effector pathways likely present rational target opportunities for therapeutic intervention. The LKB1 gene encodes a serine/threonine kinase regulating cell growth, polarity, and metabolism. An important function of LKB1 is negatively regulating a family of three CREB transcriptional co-activators (CRTC1, 2,3), which have crucial roles in metabolism, aging and cancer. We previously discovered that LKB1 loss causes enhanced levels of dephosphorylated CRTCs that subsequently translocate to the nucleus and promote transcription of CREB-dependent genes in cancer cells. However, the importance of this aberrantly active CRTC- CREB signaling axis and its underlying mechanisms in lung cancer remain poorly characterized and such knowledge will be crucial in uncovering new therapeutic strategies. Therefore, our proposed research is aimed to bridge this significant gap by elucidating this LKB1 inactivation-induced signaling for its significance and mechanisms in lung cancer. Building on our published and preliminary data, we hypothesize that aberrant CRTC activation is a core driver event that underlies LKB1 loss in lung malignancies, presenting a unique vulnerability of LKB1-inactivated lung cancers. This hypothesis will be tested by two specific aims. Aim 1 will elucidate the functional significance of aberrantly activated CRTC-CREB signaling in lung cancers with LKB1 inactivation, and Aim 2 will define the mechanisms of aberrant CRTC-CREB activation in lung cancers with LKB1 inactivation. The successful completion of these proposed studies will uncover new mechanistic and functional insights into aberrant CRTC activation in the development and progression of LKB1-inactivated lung cancer. We anticipate that these efforts will validate CRTCs as a therapeutic target, reveal novel therapeutic strategies, and contribute to our mechanistic understanding of the biology of cancer with LKB1 inactivation.